It is frequently necessary to measure or monitor the level of liquid in a tank or container. This has commonly been done by means of a float-type device.
However, it is also known to measure the liquid level by means of a plurality of optical sensors, which are positioned on the tank wall at various vertically-spaced elevations. Basically, a transparent body is provided with a conical or prismatic tip end portion. Light is propagated within the body toward the tip end portion, and is reflected by two surfaces at the tip end portion back toward a receiver. The body is typically glass and has a refractive index of about 1.50. If the tip end portion is exposed to air above the surface of the liquid, the "critical angle", at which all light is reflected within the body, may be calculated from the equation: EQU sin .theta..sub.c =n.sub.2 /n.sub.1 ( 1)
where .theta..sub.c is the "critical angle", n.sub.2 is the index of refraction of the fluid (i.e., air) to which the tip is exposed, and n.sub.1 is the index of refraction of the material (i.e., glass) of the tip end portion. Thus, for air, n.sub.2 =1.00; and for glass, n.sub.1 =1.50. Hence, this equation may readily be solved, and the "critical angle" for a glass body with respect to air is about 42.degree.. On the other hand, if the tip end portion is submerged in a liquid, such as water (i.e., n=1.33), then the "critical angle" with respect to water is about 62.5.degree..
If the prismatic surfaces are ground such that the angles of incidence are equal to or greater than the "critical angle", then all of the light propagated along the sensor body will be reflected internally of the body, and no portion of such incident light will be refracted into the surrounding fluid. This is known as the phenomenon of "total internal reflection". On the other hand, if the angle of incidence is less than the "critical angle", then only a portion of the light will be reflected, with the balance being refracted into the fluid surrounding the tip end portion.
Upon information and belief, such prior art devices have been commonly formed such that the angle of incidence (.theta..sub.i) is about 45.degree.. Hence, incident light strikes a first surface, is reflected to a second surface, and is thereafter reflected back through the body in a direction substantially parallel to the incident light beam. At the same time, such angle (i.e., 45.degree.) is greater than the "critical angle" for air/glass (i.e., .theta..sub.c =42.degree.), such that "total internal reflection" occurs when the tip end portion is exposed to air. However, if the tip end portion is submerged in water, the angle of incidence is less than the "critical angle" for water/glass (i.e., .theta..sub.c =62.5.degree.). Hence, an incident light beam will be twice refracted into the liquid. The first refraction would occur at the first surface, and the second at the second surface.
This principle (i.e., that "total internal reflection" occurs if the tip end portion is exposed to air, but that light is refracted if the tip end portion is submerged in liquid) has been used to measure the level of liquid in a tank. See, e.g., Rakucewicz, "Fiber-Optic Methods of Level Sensing", Sensors (Dec. 1986) [at p. 5 et seq.]. As indicated above, a plurality of such sensors are typically mounted on the tank wall at various vertically-spaced locations. Those sensors which are arranged above the liquid level will reflect light at a greater intensity than those which are submerged in the liquid. Thus, depending on the intensity of the internally-reflected light, one can infer whether such sensor is above or below the liquid surface.
However, in some instances, the serviced fluid may chemically attack the material of which the sensor body is made. For example, one common optical material, glass, is chemically attacked by hydroflouric acid (HF). Another common material, sapphire, is chemically attacked by sulfuric acid (H.sub.2 SO.sub.4). Thus, these conventional materials are unsuited for use in such a hostile environment.
Such corrosive fluids (e.g., hydroflouric acid, sulfuric acid, and the like) are commonly contained within tanks made of a polytetrafluoroethylene perfluoroalkoxy material, commonly known as "Teflon.RTM. PFA", manufactured by E. I. du Pont de Nemours & Co. (Inc.), Wilmington, Del. 19898. This material is relatively insensitive to chemical attack by many corrosive liquids. Because of this, upon information and belief, others have proposed to make a sensor body from such Teflon.RTM. PFA material. Indeed, it has been specifically proposed to formulate a sensor body of such material, but having the conventional 45.degree. prismatic tip end portion. This solution, while perhaps adequately addressing the problem of chemical attack, is believed to be inoperable with respect to certain serviced fluids. The reason for this is that such Teflon.RTM. PFA material has an index of refraction of approximately 1.35. Hence, the "critical angle" with respect to air would be about 48.degree., whereas the "critical angle" with respect to, say, water (i.e., n=1.33 would be about 80.degree.). In other words, if one were to make an optical sensor of such Teflon.RTM. PFA material, having an angle of incidence of 45.degree. on each inclined tip end surface, the angle of incidence would always be less than the "critical angle" with respect to air (i.e., .theta..sub.i =45.degree. &lt;.theta..sub.c =48.degree.), and with respect to water (i.e., .theta..sub.i =45.degree.&lt;.theta..sub.c =80.degree.), and light would always be refracted into the surrounding fluid regardless of whether the tip end portion was exposed to air or water. Hence, such a device would not employ the principle of "total internal reflection" to selectively provide a reflected signal of one intensity when the tip end portion is surrounded by air, but a reflected signal of lesser intensity, due to refraction, when the tip end portion is surrounded by liquid. Moreover, whereas typical optically-transmissive materials, such as glass, sapphire, and the like are relatively transparent, Teflon.RTM. PFA materials is milky or cloudy in color. Thus, some light passing through such material will be diffused, thereby further diminishing the intensity of the internally-reflected beam.
Thus, upon information and belief, the problem of providing an acceptable optical liquid level detector, particularly for use with corrosive liquids which might chemically attack the sensor body, has persisted.